Negative active material, method of preparing negative active material and lithium ion battery comprising the same

ABSTRACT

A negative active material, a method of preparing the negative active material and a lithium ion battery comprising the negative active material are provided. The negative active material may comprise: a core ( 1 ) composed of a carbon material; and a plurality of composite materials ( 2 ) attached to a surface of the core ( 1 ), each of which may comprise a first material ( 21 ) and a second material ( 22 ) coated on the first material ( 21 ), in which the first material ( 21 ) may be at least one selected from the elements that may form an alloy with lithium, and the second material ( 22 ) may be at least one selected from the group consisting of transition metal oxides, transition metal nitrides and transition metal sulfides.

CROSS-REFERENCE TO RELATED APPLICATION

This application is the national phase application of PCT ApplicationNo. PCT/CN2011/079214, filed Aug. 31, 2011, which claims the priority toand benefits of Chinese Patent Application No. 201010565469.1 filed withthe State Intellectual Property Office of P.R. China on Nov. 30, 2010;the entire content of both is hereby incorporated by reference.

FIELD

The present disclosure generally relates to lithium ion batteries, moreparticularly relates to a negative active material, a method ofpreparing the same and a lithium ion battery comprising the same.

BACKGROUND

It is to be understood that the description in this part only providessome background information relating to the present disclosure, whichmay or may not constitute a so called prior art.

Nowadays, a lithium ion battery is known as a green chemical powersource. Compared with conventional nickel-cadmium batteries andconventional nickel hydrogen batteries, the lithium ion battery has manyadvantages such as higher voltage, longer cycling life and higher energydensity. Ever since the Japanese company Sony launched the firstgeneration lithium ion batteries, lithium ion batteries have beendeveloped rapidly and applied in kinds of portable devices. The negativeelectrode in the conventional lithium ion battery comprises graphitecarbon material. However, the theoretical specific capacity of thecarbon material is about 372 mAh/g. Therefore, further improvement ofthe capacity of the lithium ion battery is restricted.

SUMMARY

The present disclosure is directed to solve at least one problemexisting in the prior art. Accordingly, a negative active material for alithium ion battery and a method for preparing the same may need to beprovided, which may enhance the cycling performance of the negativeactive material. Further, a lithium ion battery may also need to beprovided.

Some embodiments of the present disclosure provide a negative activematerial for a lithium ion battery, comprising: a core composed of acarbon material; and a plurality of composite materials attached to asurface of the core, each of which comprises a first material and asecond material coated on the first material, in which the firstmaterial is at least one selected from the elements that can form analloy with lithium, and the second material is at least one selectedfrom the group consisting of transition metal oxides, transition metalnitrides and transition metal sulfides.

Some embodiments of the present disclosure provide a method of preparinga negative active material for a lithium ion battery, comprising thesteps of: preparing a plurality of composite materials by coating asecond material onto a first material, the first material being at leastone selected from the elements that can form an alloy with lithium, andthe second material being at least one selected from the groupconsisting of transition metal oxides, transition metal nitrides andtransition metal sulfides; and bonding the plurality of compositematerials onto the surface of a carbon material layer, whereby obtainingthe negative active material.

Some embodiments of the present disclosure further provide a lithium ionbattery, comprising: a battery shell; and an electrical core and anelectrolyte received in the battery shell, the electrical corecomprising a positive electrode, a negative electrode and a separatordisposed therebetween, the negative electrode comprising a substrate andthe above mentioned negative active material bonded on the substrate.

The negative active material according to an embodiment of the presentdisclosure has a special structure, in which a first material is coatedwith a second material to form a composite material, and the compositematerial is then attached onto the exterior surface of the core to formthe special structure. The core composed of the carbon material may beused as a skeleton of the negative active material, thus effectivelyavoiding agglomeration of nanometer materials and providing excellentelectron conductive channels. The intermediate layer composed of thefirst material is coated onto the surface of the core, and the firstmaterial may be at least one material that can form an alloy withlithium, thus ensuring high capacity of the negative active material.The second material forming the outmost layer coated on the surface ofthe intermediate layer may be at least one selected from the groupconsisting of transition metal oxides, transition metal nitrides andtransition metal sulfides; and the transition metal oxides, transitionmetal nitrides or transition metal sulfides may form a dynamic solidelectrolyte layer during charging and discharging, thus furtherimproving the cycling performance of the negative active material andeffectively avoiding side reactions caused by volume change of the firstmaterial in the charging and discharging process.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present disclosure will be readily appreciated,as the same becomes better understood by reference to the followingdetailed description when taken in conjunction with the accompanyingdrawings, in which:

The drawing of the present disclosure is a schematic view of a negativeactive material for a lithium ion battery according to an embodiment ofthe present disclosure.

DETAILED DESCRIPTION

Reference will be made in detail to embodiments as well as the drawingsof the present disclosure. The embodiments described herein withreference to drawings are explanatory, illustrative, and used togenerally understand the present disclosure. The embodiments shall notbe construed to limit the present disclosure. And it should beunderstood that any appropriate design modification or any improvementand the like which can be made based on the knowledge of those skilledin the art without departing from the spirit is within the scope of thepresent disclosure.

As shown in the drawing of the present disclosure, the negative activematerial according to some embodiments of the present disclosurecomprises: a core 1 and a plurality of composite materials 2 attachedonto the surface of the core 1, the core 1 may be composed of a carbonmaterial, the composite materials 2 may comprise a first material 21 anda second material 22 coated onto the first material 21, the firstmaterial 21 may be at least one selected from the elements which canform alloys with lithium, and the second material 22 may be at least oneselected from the group consisting of transition metal oxides,transition metal nitrides and transition metal sulfides.

The inventors of the present disclosure have found, after long-timeexperimentation, that the negative active material composed oftransition metal oxides, transition metal nitrides or transition metalsulfides have a lithium storage mechanism different from those of aninsert type negative active material (e.g., the carbon material) and analloy type negative active material (e.g., the first material). Suchmaterials form lithium oxides, nitrides or sulfides in the lithiuminsertion process, which may improve the stability of the electrolytefilm. The negative active material according to an embodiment of thepresent disclosure has a special structure, in which a first material iscoated with a second material to form a composite material, and thecomposite material is then attached onto the exterior surface of thecore to form the special structure. The core composed of the carbonmaterial may be used as a skeleton of the negative active material, thuseffectively avoiding agglomeration of nanometer materials and providingexcellent electron conductive channels. The intermediate layer composedof the first material is coated onto the surface of the core, and thefirst material may be at least one material that can form an alloy withlithium, thus ensuring high capacity of the negative active material.The second material forming the outmost layer coated on the surface ofthe intermediate layer may be at least one selected from the groupconsisting of transition metal oxides, transition metal nitrides andtransition metal sulfides; and the transition metal oxides, transitionmetal nitrides or transition metal sulfides may form a dynamic solidelectrolyte layer during charging and discharging, thus furtherimproving the cycling performance of the negative active material andeffectively avoiding side reactions caused by volume change of the firstmaterial in the charging and discharging process.

In one embodiment, the composite materials are distributed on theexternal surface of the core at intervals. In a preferred embodiment,the composite materials are uniformly distributed on the externalsurface of the core, and the intervals between the adjacent compositematerials are about 0.1 μm to about 2 μm. The interval distribution ofthe composite materials may provide more electron conductive channelsand electrolyte infiltration paths, thus improving the rate performanceand the low temperature performance of the battery.

In the negative active material of the present disclosure, the carbonmaterial may be any carbon material known in the art that is able toreversibly intercalate and de-intercalate lithium ions, for example,natural graphite, artificial graphite, coke, carbon black, pyrolyticcarbon, or carbon fiber. In an embodiment, the carbon material may be atleast one selected from graphite, hard carbon, soft carbon, graphizedmesocarbon microbeads (MCMB), carbon fiber, carbon nanotubes.

To achieve better lithium ion intercalation and de-intercalationperformance of the carbon material, in a preferred embodiment, each ofgraphite, hard carbon, soft carbon, and MCMB may have an averageparticle diameter of about 2 μm to 20 μm, and each of carbon fiber andcarbon nanotube may have an average particle diameter of about 10 nm to500 nm and a length of about 2 μm to 50 μm. Carbon material as theskeleton of the negative active material may effectively avoid theagglomeration of the first material and the second material and mayprovide excellent electron conductive channels. The core 1 may have aspherical shape shown in the drawing of the present discloure. It is tobe understood that the core 1 may not only have a spherical shape butalso may have other shapes, for example, a near-spherical shape, such asan ellipsoidal shape, a sheet-like shape, a linear shape, or athree-dimensional reticular shape.

The first material may be at least one selected from the elements thatcan form an alloy with lithium. For example, the first material may beat least one selected from Si, Ge, Sn, Sb, Al, Pb, Ga, In, Cd, Ag, Hgand Zn, and the median particle size of the first material is about 50nm±10 nm. As these elements may form an alloy with lithium, the capacityof the negative electrode in the lithium ion battery may be enhanced. Ina preferred embodiment, in order to further enhance the capacity of thenegative electrode, the first material may be at least one selected fromthe group consisting of Si, Ge, Sn and Sb. The first material may beformed into a sheet-like shape as shown in the drawing of the presentdisclosure. It is to be understood that the first material may be formedinto other shapes, for example, a spherical shape, a near-sphericalshape, or a linear shape.

The second material may be at least one selected from transition metaloxides, transition metal nitrides and transition metal sulfides. Forexample, the second material may be at least one selected from thecompounds formed by at least one element of O, N and S with at least oneelement of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh,Pd, Ag, Cd, La, Hf, Ta, Re, W, Os, Ir, Pt, Au and Hg. Preferably, theaverage particle diameter of the second material is less than 1 μm sothat it may be beneficial to the formation of a compact second materiallayer. It can be understood that the second material may be formed intoa shape that is corresponding to that of the first material, and may beformed into different shapes, for example, a hollow spherical shape, ahollow near-spherical shape, or a three-dimensional reticular shape.There is no special limit on the shape of the second material layer,provided that the second material is continuously coated onto theexterior surface of the first material and forms a composite material.

In the negative active material of the present disclosure, in someembodiments, base on the total weight of the negative active material,the amount of the first material may be about 2 wt % to 50 wt %, theamount of the second material may be about 0.1 wt % to 20 wt %, and theresidual may be the carbon material. In a preferred embodiment, base onthe total weight of the negative active material, the amount of thefirst material may be about 5 wt % to 20 wt %, the amount of the secondmaterial may be about 1 wt % to 5 wt %, and the residual may be thecarbon material. By using the preferred composition of the negativeactive material, the specific capacity and the cycling performance ofthe negative electrode may be improved accordingly.

The negative active material has a special structure, in which a firstmaterial is coated with a second material to form a composite material,and the composite material is then attached onto the exterior surface ofthe core to form the special structure. The existence of the secondmaterial may enhance the cycling performance of the negative activematerial. The reasons lie in that the second material may form a dynamicsolid electrolyte layer during charging and discharging, thuseffectively avoiding side reactions caused by the volume change ofmaterials like Si, Ge, Sn or Sb during charging and discharging.

Specifically, in the charging process, the second material is firstlyreacted with lithium ions to form lithium oxides, lithium nitrides orlithium sulfides in the early stage of the charging process. Thesematerials may form a solid electrolyte layer, which is an ion conductorand allows lithium ions to pass therethrough. Meanwhile, these materialsmay decrease the conductivity of electrons on the surface of thenegative electrode, thus effectively preventing the decomposition ofsolvent molecules after obtaining electrons from the surface of thenegative electrode. With the progress of the charging process, thelithium ions may react with the first material and the carbon materialin the core. Although the volume of the first material and the carbonmaterial may be largely increased in the reaction, the formation of theion conductor in the early stage of the charging process may preventlarge scale breaking and/or peeling off of the materials like Si, Ge,Sn, or Sb in the volume expansion process, and may restrain the unwantedresults caused by the volume expansion. Further, during discharging, thelithium ions may be firstly de-intercalated from the core and the firstmaterial layer, and with the extraction of the lithium ions, lithiumoxides, lithium nitrides or lithium sulfides may form transition metaloxides, transition metal nitrides or transition metal sulfidesrespectively and the negative active material may be recovered to itsinitial state. In this way, the side reaction of the electrolyte withthe expanded and pulverized alloy material such as Si, Ge, Sn, or Sb maybe effectively avoided, thus avoiding problems such as electronconduction blocking between the materials caused by by-products.

It can be understood that the present disclosure is not restricted tothe embodiments of the present disclosure. Other negative activematerials of different structures may also be applied in the presentdisclosure as long as the purpose and effect of the present disclosurecan be realized.

Embodiments of the present disclosure further provide a method ofpreparing the negative active material as described above. In oneembodiment, a first material is coated with a second material to preparea composite material, and then the composite material is distributedonto the surface of a carbon material. Various methods in the art may beused to prepare the negative active material.

In an alternative embodiment, the preparation process comprises thefollowing steps.

Step 1. To a high pressure reaction container of about 2.5 L to 3 L witha PTFE inner lining, distilled water and a transition metal salt areadded in turn and agitated to form a uniform solution; a dispersant isadded under intense agitation; then a first material is added, anaccelerant is added with stifling; then an appropriate amount ofdistilled water is added until about 60% to 95% of the total volume ofthe reaction container is occupied and then the reaction container issealed; with continuously stirring, the temperature of the solution isincreased to about 110-200° C. and maintained for about 12 hours to 24hours to obtain a precursor suspension. The precursor suspension isfiltered, washed and then dried to obtain a solid product, then thesolid product is placed in a speckled furnace and calcined in an airatmosphere at about 300-900° C. for about 4 hours to 12 hours to obtaina composite material formed by coating a second material on a firstmaterial.

The above steps may be performed in a reaction container, and thecomposite material may be prepared by a hydro-thermo process. Thereaction container may provide a sealed environment for agitating. Undernormal pressure, the water solution will be boiled at about 100° C. Whenin a sealed condition with a higher pressure, the water solution willhave a higher boiling point so that the water solution will have ahigher temperature.

The second material may be at least one selected from transition metaloxides, transition metal nitrides and transition metal sulfides. Forexample, the second material may be at least one selected from thecompounds formed by at least one of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn,Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, La, Hf, Ta, Re, W, Os, Ir, Pt, Auand Hg with at least one of O, N and S.

In the present step, transition metal salts are used as startingmaterials. For example, chlorides, nitrates or sulfates of a transitionmetal may be used to prepare oxides of the transition metal. In oneembodiment, the second material is Co₃O₄, and the transition metal saltsmay be CoCl₂.6H₂O, Co(NO₃)₂.6H₂O or CoSO₄.6H₂O. The outermost layer ofthe prepared composite material in Step 1 is composed of a transitionmetal oxide, and the preparation method of the outermost layer is thein-situ synthesis method. To prepare a composite material with anoutermost layer of transition metal nitrides or transition metalsulfides, nano-particles of transition metal nitrides or transitionmetal sulfides may be used to coat the composite material obtained inStep 1, and the methods may be any known in the art, and thus thedescription thereof is omitted herein for brevity.

The first material may be at least one selected from elements that canform an alloy with lithium. To improve the capacity of the negativeelectrode in the lithium ion battery, the first material may be at leastone selected from the group consisting of Si, Ge, Sn and Sb.

The dispersant may be polyethyleneglycol or polyvinylpyrrolidone, towell disperse the second material salt. The accelerant may be selectedfrom ammonia water, ammonium oxalate and sodium tartrate to promote theformation of the precursor suspension.

In an embodiment, base on 100 weight parts of the first material, theamount of the second material may be about 0.2 weight parts to 100weight parts, the amount of the dispersant may be about 2 weight partsto 20 weight parts, and the amount of the accelerant may be about 2weight parts to 30 weight parts.

Step 2. The carbon material and the composite material in step 1 areadded to an organic solvent, then a non-water-soluble polymer is addedto the solvent with intensely stirring to form a stable solid-liquidmixture. The solid-liquid mixture is dried under vacuum at about 50-100°C., then the dried product is calcined at about 300-900° C. for 4 hoursto 12 hours in an inert gas protection to obtain the negative activematerial.

Calcining under inert gas protection may prevent oxidation of the carbonmaterial and help the non-water-soluble polymer to form an amorphouscarbon layer on the surface of the carbon material and the compositematerial, and therefore the obtained negative active material may have amore stable structure.

The carbon material may be any carbon material known in the art that isable to reversibly intercalate and de-intercalate lithium ions, forexample, natural graphite, artificial graphite, coke, carbon black,pyrolytic carbon, or carbon fiber. In a preferred embodiment, the carbonmaterial may be at least one selected from graphite, hard carbon, softcarbon, graphized mesocarbon microbeads (MCMB), carbon fiber and carbonnanotubes. To achieve better insertion and extraction (or intercalationand de-intercalation) effect of the carbon material, preferably, each ofthe graphite, hard carbon, soft carbon and graphized MCMB has an averageparticle diameter of about 2 μm to 20 μm, and each of carbon fiber andcarbon nanotube has an average particle diameter of about 10 nm to 500nm and a length of about 2 μm to 50 μm. Carbon material as the skeletonof the negative active material may effectively prevent theagglomeration of the first material and the second material and providefavorable electron conducting channels.

The organic solvent may be at least one selected from ethanol, acetone,tetrahydrofuran and N-methylpyrrolidone.

The non-water-soluble polymer is used to stably attach the compositematerial onto the surface of the carbon material. In some embodiments,the non-water-soluble polymer may be at least one selected from thegroup consisting of polythiophene, polypyrrole, polytetrafluoroethylene,polyvinylidene fluoride, polyethylene, polypropylene,ethylene-propylene-diene copolymer resin, styrene-butadiene rubber,polybutadiene, fluororubber, polyethylene oxide, polyester resin,phenolic resin, epoxy resin, carboxypropyl cellulose, ethyl celluloseand asphaltum.

In a preferred embodiment, based on 100 weight parts of the compositematerial, the amount of the carbon material is about 100 weight parts to2000 weight parts, the amount of the organic solvent is about 100 weightparts to 4000 weight parts, and the amount of the non-water-solublepolymer is about 2 weight parts to 20 weight parts.

Embodiments of the present disclosure provide a lithium ion batteryincluding the above negative active material, comprising: a batteryshell; and an electric core and an electrolyte received in the shell.The electric core may comprise a positive electrode, a negativeelectrode, and a separator disposed between the positive electrode andthe negative electrode. The negative electrode may comprise a negativesubstrate and a negative active material disposed on the negativesubstrate, in which the negative active material is the negative activematerial described above.

In some embodiments, the negative electrode may be prepared by coating aslurry containing the negative active material on the negativesubstrate. The negative substrate may be a copper foil. To a basematerial, the above negative active material may be added at apredetermined ratio, and a binding agent and a solvent may be furtheradded and mixed uniformly to form a slurry. The slurry is then coated onthe copper foil to form the negative electrode. There is no speciallimit on the binding agent and the solvent, and any binding agent andany solvent known in the art may be used. In some embodiments, thebinding agent may be CMC (sodium carboxymethyl cellulose), and thesolvent may be SBR (styrene-butadiene rubber).

The positive electrode may comprise a positive substrate and a positiveactive material disposed on the positive substrate. In one embodiment,the positive substrate may be an aluminum foil or a nickel screen. Thepositive active material may be metal sulfides or oxides. For example,the positive active material may be at least one selected from the groupconsisting of TiS₂, MoS₂, V₂O₅ and lithium composite oxides. There is nospecial limit on the preparation method of the positive electrode, andthe slurry coating method may also be used. The positive material maynot only comprise a binding agent and a solvent, but also comprise aconductive agent.

There is no special limit on the electrolyte, and any electrolyte knownin the art may be used. For example, the electrolyte may be at least oneselected from LiPF₆, LiBF₄, LiAsF₆, LiClO₄, LiCH₃SO₃, LiN(SO₂CF₃)₂,LiC(SO₂CF₃)₃, LiAlCl₄, LiSiF₆, LiB(C₆H₅)₄, LiCl and LiBr.

There are no special limits on the battery shell, the positiveelectrode, the electrolyte and the separator, and any battery shell, anypositive electrode, any electrolyte and any separator known in the artmay be used. The preparation method of the electric core may be anymethod known in the art and the structure of the electric core may beany structure known in the art, for example, a winding structure, or anoverlapping structure.

The present disclosure will be further described in details inconjunction with the detailed examples. And it should be understood thatthe detailed examples below are only used to explain instead of limitingthe present disclosure.

Example 1 (1) Preparation of Composite Material

A. To a 3 L high pressure reaction container with apolytetrafluoroethylene inner lining, 2000 g of distilled water and 20 gof CoCl₂.6H₂O were added and agitated to form a uniform solution; 10 gof polyethyleneglycol 6000 dispersant was then added to the solutionunder intense agitation; then 100 g of nano silicon powder with a medianparticle size of about 50 nm was added and the solution was agitated,with agitating, 15 g of ammonium oxalate was added; afterwards,distilled water was added until 80% of the total volume of the reactioncontainer was filled and the reaction container was then sealed. Thetemperature was increased to 180° C. with agitating and then maintainedfor 24 hours to obtain a precursor suspension. The precursor suspensionwas filtered, washed and dried to obtain a solid product, and theobtained solid product was placed in a speckled furnace at a temperatureof about 400° C. and calcined in an air atmosphere for about 6 hours toobtain a nanosilicon-Co₃O₄ composite material.

B. 900 g of graphite and 120 g of nanosilicon-Co₃O₄ composite materialwere added to 2000 g of an organic solvent of acetone andtetrahydrofuran with a weight ratio of 1:1, to the solvent was thenadded 10 g of modified polyvinylidene fluoride; the solution wasintensely agitated to form a uniform and stable solid-liquid mixture,the mixture was stirred at about 80° C. for 3 hours until most of thesolvent was vaporized to obtain a solid product, and the obtained solidproduct was dried at about 120° C. under vacuum and calcined under inertgas protection at a temperature of about 500° C. for 8 hours to obtain acomposite material S1. The composite material S1 has a core composed ofgraphite, and the composite material is composed of nano silicon andCo₃O₄ coated nano silicon.

(2) Preparation of Button Cell

The composite material S1 prepared in step 1 was used, and according tothe weight ratio of S1:CMC:SBR=100:2:3, S1, CMC and SBR were mixeduniformly and pressed to prepare a negative plate. Then, the negativeplate was placed in a 120° C. oven and dried under vacuum for above 12hours. Afterwards, in a glove box with an argon atmosphere, the negativeplate as a working electrode and the metal lithium as a counterelectrode were assembled to form a button cell A1.

(3) Preparation of Whole Cell

The composite material S1 as the negative active material was mixed withthe binding agent CMC, SBR and water to prepare a negative material. Thelithium cobaltate as the positive active material was mixed with thebinding agent PVDF, a conductive agent, acetylene black and water toprepare a positive material. The positive and negative materials werecompounded, coated, dried, rolled and sliced to obtain positive andnegative plates respectively. The positive plate, the negative plate,and a polypropylene separator having a thickness of 20 μm were wound toform a square electric core. The electric core was disposed and sealedin a 5 mm×34 mm×50 mm square aluminum shell to form a 053450 typelithium ion battery. After injecting an electrolyte of 1 mol/L LiPF₆solution of FEC/DEC with a volume ratio of 4/6, laying aside and aging,forming and capacity grading, a whole cell B1 was obtained.

Example 2

The method for preparing a composite material S2 is substantially thesame as that in Example 1, except that: CuCl₂.6H₂O was used tosubstitute for CoCl₂.6H₂O, and ammonia water was used to substitute forammonium oxalate. The amount of CuCl₂.6H₂O was 15 g, and the amount ofthe ammonia was about 10 g. Distilled water was added until 90% of thetotal volume of the reaction container was filled and the reactioncontainer was then sealed. The temperature of the mixture in thereaction container was increased to about 120° C. with agitating andthen maintained for about 12 hours to obtain the precursor suspension.After the precursor suspension was filtered, washed and dried to obtaina solid product, the obtained solid product was placed in a speckledfurnace at a temperature of about 400° C. and calcined under an argonatmosphere for 6 hours to obtain a composite material S2 of copper oxidecoated nano silicon.

Example 3

The method for preparing a composite material S3 is substantially thesame as that in Example 1, except that: nano tin powder was used toreplace nano silicon power as the first material and calcined at atemperature of 320° C. to obtain a composite material S3 of Co₃O₄ coatednano tin.

A button cell A3 and a whole cell B3 were prepared according to theprocesses the same as that in Example 1.

Example 4

The method for preparing a composite material S4 is substantially thesame as that in Example 1, except that: nano zinc powder was used toreplace nano silicon power as the first material and calcined at atemperature of 320° C. to obtain a composite material S4 of Co₃O₄ coatednano zinc.

A button cell A4 and a whole cell B4 were prepared according to theprocesses the same as that in Example 1.

Example 5

The method for preparing a composite material S5 is substantially thesame as that in Example 1, except that: Co(NO₃)₂.6H₂O was used tosubstitute for CoCl₂.6H₂O, ammonia water was used to substitute forammonium oxalate, the amount of Co(NO₃)₂.6H₂O was 25 g, and the amountof the ammonia water was about 10 g. Distilled water was added until 90%of the total volume of the reaction container was filled and thereaction container was then sealed. The temperature of the mixture inthe reaction container was increased to about 120° C. with agitating andthen maintained for about 24 hours to obtain a precursor suspension.After the precursor suspension was filtered, washed and dried to obtaina solid product, the obtained solid product was placed in a speckledfurnace at a temperature of about 450° C. and calcined in an argonatmosphere for 6 hours to obtain a composite material S5 of cobalt oxidecoated nano silicon.

A button cell A5 and a whole cell B5 were prepared according to theprocesses the same as that in Example 1.

Example 6

The method for preparing a composite material S6 is substantially thesame as that in Example 1, except that: NiCl.6H₂O was used to replaceCoCl₂.6H₂O and a composite material S6 of Ni₂O coated nano silicon wasprepared.

A button cell A6 and a whole cell B6 were prepared according to theprocesses the same as that in Example 1.

Comparative Example 1 (1) Preparation of Composite Material

900 g of graphite and 100 g of nano silicon powder with a medianparticle size of about 50 nm were added to an organic solvent of acetoneand tetrahydrofuran with a weight ratio of 1:1, 10 g of modifiedpolyvinylidene fluoride was then added; and the mixture was mixed andagitated to form a stable solid-liquid mixture. The solid-liquid mixturewas agitated at 80° C. for 3 hours until most of the solvent wasvaporized. The residual was dried at 120° C. under vacuum to obtain agraphite-nano silicon composite material SC1.

(2) Preparation of Button Cell

According to the weight ratio of SC1:CMC:SBR=100:2:3, SC1, CMC and SBRwere mixed uniformly and pressed to prepare a negative plate. Then, thenegative plate was placed in a 120° C. oven and dried under vacuum forabove 12 hours. Afterwards, in a glove box with an argon atmosphere, thenegative plate as a working electrode and the metal lithium as a counterelectrode were assembled to form a button cell AC1.

(3) Preparation of Whole Cell

The composite material S1 as the negative active material was mixed withthe binding agent CMC, SBR and water to prepare a negative material. Thelithium cobaltate as the positive active material was mixed with thebinding agent PVDF, a conductive agent, acetylene black and water toprepare a positive material. The positive material and the negativematerial were compounded, coated, dried, rolled and sliced to obtainpositive and negative plates respectively. The positive plate, thenegative plate, and a polypropylene separator having a thickness of 20μm were wound to form a square electric core. The electric core wasdisposed and sealed in a 5 mm×34 mm×50 mm square aluminum shell toprepare a 053450 type lithium ion battery. After injecting anelectrolyte of 1 mol/L LiPF₆ solution of FEC/DEC with a volume ratio of4/6, laying aside and aging, forming and capacity grading, a whole cellBC1 was obtained.

Comparative Example 2

To a 3 L high pressure reaction container with a polytetrafluoroethyleneinner lining, 2000 g of distilled water and 20 g of CoCl₂.6H₂O wereadded and agitated to form a uniform solution; 10 g ofpolyethyleneglycol 6000 dispersant was then added to the solution underintense agitation; then 100 g of nano silicon powder with a medianparticle size of about 50 nm was added and the solution was agitated,with agitating, 15 g of ammonium oxalate was added; afterwards,distilled water was added until 80% of the total volume of the reactioncontainer was filled and the reaction container was then sealed. Thetemperature was increased to 180° C. with agitating and then maintainedfor 24 hours to obtain a precursor suspension. The precursor suspensionwas filtered, washed and dried to obtain a solid product, and theobtained solid product was placed in a speckled furnace at a temperatureof about 400° C. and calcined in an air atmosphere for about 6 hours toobtain a nanosilicon-Co₃O₄ composite material SC2.

A button cell AC2 and a whole cell BC2 were prepared according to theprocesses the same as that in Comparative Example 1.

Performance Test

(1) Specific Capacity Test

20 pieces of each of button cells A1 to A6 and AC1 to AC2 were taken andtested on the Blue-key BK-6016 secondary battery performance testingdevice at a temperature of about 25±1° C. to obtain the battery capacitythereof. The testing steps are as follows: laying aside for 30 min;discharging the battery to 0.005V at a constant current of about 0.2 mA;discharging the battery to 0.005V at a constant current of about 0.1 mA;discharging the battery to 0.005V at a constant current of about 0.05mA; then laying aside for 10 minutes; and then charging the battery at acurrent of 0.2 mA to 2.5V. Then, the specific capacity of the activematerial was calculated according to the following equation: specificcapacity of the active material=tested battery capacity/weight of theactive material in the button cell, and the averages of the specificcapacities were recorded.

The results were shown in Table 1. Please be noted that the lithiuminsertion specific capacity is the total specific capacity in thedischarging step, and the lithium extraction specific capacity is thetotal specific capacity in the charging step.

TABLE 1 Cell A1 A2 A3 A4 A5 A6 AC1 AC2 Lithium insertion specific 731677 708 432 710 708 726 2320 capacity/mAh/g Lithium extraction specific606 583 602 387 600 602 596 1234 capacity/mAh/g Initial efficiency/%82.9 86.1 85.0 89.6 84.5 85.0 82 53.2

(2) Cycling Test

20 pieces of each of whole cells B1-B6 and BC1 to BC2 were tested on theKinte BS-9300 secondary battery testing device, and at a temperature ofabout 25±1° C., the cycling performance of the whole cell was tested at0.2 C. The steps are as follows: laying aside the battery for 10 min;charging the battery at a constant voltage to 4.2V/0.05 C; laying asidefor 10 minutes; and then discharging the battery to 3.0V at a constantcurrent. The above steps form 1 cycle. The steps are repeated until thebattery capacity is lower than 80% of the initial discharging capacity.The cycle times are recorded as the battery cycling life, and an averagevalue was taken in each group. The results are shown in Table 2.

TABLE 2 Battery B1 B2 B3 B4 B5 B6 BC1 BC2 Cycling life/times 432 358 422355 367 295 42 5 Battery internal resistance 65 62 68 63 61 61 95 120after cycling/mΩ

It can be seen from Tables 1 and 2 that, the negative active materialsaccording to Examples 1 to 6 have a higher specific capacity (the carbonmaterial has a specific capacity of about 372 mAh/g), and although thespecific capacities of the negative active materials according toExample 1 to Example 6 are lower than that of the negative activematerial according to Comparative Example 2, the cycling performance ofthe negative active materials according to Example 1 to Example 6 aremuch better than those of the negative active materials prepared inComparative Example 1 and Comparative Example 2. From above, the cyclingperformances of the batteries made of the negative active materialaccording to the present disclosure are enhanced to a great extent.

Although explanatory embodiments have been shown and described, it wouldbe appreciated by those skilled in the art that changes, alternatives,and modifications may be made in the embodiments without departing fromspirit and principles of the disclosure. Such changes, alternatives, andmodifications all fall into the scope of the claims and theirequivalents.

What is claimed is:
 1. A negative active material for a lithium ionbattery, comprising: a core comprising a carbon material; and aplurality of composite material particles attached to a surface of thecore, each of the composite material particles comprising a firstmaterial and a second material coated on the first material; wherein thefirst material is at least one selected from the elements that can forman alloy with lithium, and the second material is at least one selectedfrom the group consisting of transition metal oxides, transition metalnitrides and transition metal sulfides.
 2. The negative active materialaccording to claim 1, wherein the plurality of composite materialparticles are distributed on an external surface of the core atintervals.
 3. The negative active material according to claim 2, whereinthe plurality of composite material particles are uniformly distributedon the external surface of the core and the intervals between thecomposite materials are about 0.1 μm to 2 μm.
 4. The negative activematerial according to claim 1, wherein the core has a spherical shape,the first material is formed into a laminated shape, and the secondmaterial is formed into a hollow spherical shape.
 5. The negative activematerial according to claim 1, wherein in the negative active material,the weight percentage of the first material is about 2 wt % to 50 wt %,and the weight percentage of the second material is about 0.1 wt % to 20wt %.
 6. The negative active material according to claim 1, wherein thecarbon material is at least one selected from the group consisting ofgraphite, hard carbon, soft carbon, graphized mesocarbon microbeads,carbon fiber and carbon nanotubes.
 7. The negative active materialaccording to claim 1, wherein the first material is at least oneselected from the group consisting of Si, Ge, Sn, Sb, Al, Pb, Ga, In,Cd, Ag, Hg and Zn.
 8. The negative active material according to claim 1,wherein the second material is at least one selected from the groupconsisting of oxides of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Nb,Mo, Tc, Ru, Rh, Pd, Ag, Cd, La, Hf, Ta, Re, W, Os, Ir, Pt, Au, and Hg.9. A method of preparing a negative active material for a lithium ionbattery, comprising the steps of: preparing a plurality of compositematerial particles by coating a second material onto a first material,the first material being at least one selected from the elements thatcan form an alloy with lithium, and the second material being at leastone selected from the group consisting of transition metal oxides,transition metal nitrides and transition metal sulfides; and attachingthe plurality of composite material particles onto the surface of acarbon material layer, whereby obtaining a negative active material. 10.The method according to claim 9, wherein the step of preparing aplurality of composite material particles further comprises: addingdistilled water and a salt of a transition metal into a reactioncontainer and stirring to form a uniform solution; adding the firstmaterial to the solution; increasing the temperature inside the reactioncontainer to obtain a solid product; and sintering the solid product toobtain the composite material particles with the second material beingcoated on the first material.
 11. The method according to claim 9,wherein the step of attaching further comprises: adding the carbonmaterial and the composite material particles to an organic solution;adding a non-water-soluble polymer to the solution; stirring thesolution to obtain a uniform and stable solid-liquid mixture; andsintering the mixture to obtain the negative active material.
 12. Themethod according to claim 9, wherein the plurality of composite materialparticles are uniformly distributed on the external surface of the coreand the intervals between the composite material particles is about 0.1μm to 2 μm.
 13. The method according to claim 9, wherein the core has aspherical shape, the first material is formed into a laminated shape andthe second material is formed into a hollow spherical shape.
 14. Themethod according to claim 9, wherein based on the total weight of thenegative material, the weight percentage of the first material is about2 wt % to 50 wt %, and the weight percentage of the second material isabout 0.1 wt % to 20 wt %.
 15. A lithium ion battery, comprising: abattery shell; and an electrical core and an electrolyte in the batteryshell, the electrical core comprising a positive electrode, a negativeelectrode and a separator disposed therebetween; the negative electrodecomprising a substrate and a negative active material; wherein thenegative active material comprises: a core comprising a carbon material;and a plurality of composite material particles attached to a surface ofthe core, each of the composite material particles comprising a firstmaterial and a second material coated on the first material; wherein thefirst material is at least one selected from the elements that can forman alloy with lithium, and the second material is at least one selectedfrom the group consisting of transition metal oxides, transition metalnitrides and transition metal sulfides.
 16. The negative active materialaccording to claim 1, wherein the carbon material is at least oneselected from the group consisting of graphite, hard carbon, softcarbon, graphized mesocarbon microbeads, and the carbon material has anaverage particle diameter of about 2 μm to about 20 μm.
 17. The negativeactive material according to claim 1, wherein the carbon material is atleast one selected from the group consisting of carbon fiber and carbonnanotubes, and the carbon material has a particle diameter of about 10nm to about 500 nm and a length of about 2 μm to about 50 μm.
 18. Thenegative active material according to claim 1, wherein the secondmaterial has the average particle diameter of less than about 1 μm. 19.The negative active material of claim 1, wherein the first material isselected from the group consisting of Si, Zn and Sn.
 20. The negativeactive material of claim 1, wherein the second material is selected fromthe group consisting of cobalt oxide, nickel oxide, and copper oxide.